The present invention relates generally to power amplification circuits, and more particularly, to an improvement in a circuit for performing a switching operation in response to a pulse width modulation (hereinafter, referred to as PWM) signal based on an audio signal or the like so as to cause the PWM signal to be subjected to power amplification, and then supplying a circuit load including a speaker or the like with an output signal obtained with the power amplification.
In the field of acoustic apparatus operative to amplify an audio signal and supply a speaker with the amplified audio signal to obtain reproduced sound based on the audio signal, there have been proposed various systems for amplifying the audio signal in pursuit of the respective purposes assigned thereto. In particular, with respect to power amplification performed to obtain, based on an input audio signal, an output audio signal used for driving a speaker, a D-class amplification which is carried out by an active amplifying element, such as a transistor, functioning in the manner of D-class operation, is often adopted because a relatively superior distortion characteristic can be obtained thereby.
D-class amplification is generally performed with a switching operation of an active amplifying element, for example, a transistor, in response to an input signal which is an audio signal or the like. In the case of a power amplification circuit which performs D-class amplification for audio signals, there previously has been proposed such a circuit that is operable first to produce a PWM signal based on an input audio signal, then to cause the PWM signal to be subjected to power amplification, and to supply a speaker portion with the amplified PWM signal through a low pass filter (hereinafter, referred to as LPF), as shown in, for example, Japanese patent application published before examination under publication number 2002-158544.
The power amplification circuit previously proposed is configured to be a so-called Balanced Transformerless (hereinafter, referred to as BTL) type, namely, a power amplification circuit in which a couple of switching amplifier portions are provided for driving a speaker portion connected in common to the switching amplifier portions. In such a power amplification circuit, a pulse width modulation amplifier is provided for producing first and second PWM signals having complementary variations in the respective pulse widths caused in response to a digital signal in an audio frequency band as the input audio signal. The first PWM signal obtained from the pulse width modulation amplifier is subjected to power amplification carried out with the switching operation of a first power switching circuit (a first switching amplifier) performed in response to the first PWM signal, and a first PWM power signal which is obtained as an output signal of the first power switching circuit is supplied to a first power LPF. The second PWM signal obtained from the pulse width modulation amplifier is subjected to power amplification carried out with the switching operation of a second power switching circuit (a second switching amplifier) performed in response to the second PWM signal, and a second PWM power signal which is obtained as an output signal of the second power switching circuit is supplied to a second power LPF.
Then, a couple of signals derived, respectively, from the first and second power LPFs, which are opposite one another in polarity, are supplied to the speaker portion connected to both of the first and second power LPFs. As a result, the speaker portion is driven differentially with the signals contrary opposite one another in polarity to reproduce sound in response to the input audio signal.
In the previously proposed power amplification circuit mentioned above, the first and second PWM signals obtained from the pulse width modulation amplifier based on the digital signal in the audio frequency band as the input audio signal are produced, for example, with the use of a common clock signal and therefore have the respective periods synchronized with one another. Accordingly, the rising or falling edge at the beginning of each period of the first PWM signal appears at substantially the same time as the rising or falling edge at the beginning of each period of the second PWM signal, disregarding extremely slight accidental time differences. This means that the switching operation of the first power switching circuit in response to the rising or falling edge at the beginning of each period of the first PWM signal is performed at substantially the same time as the switching operation of the second power switching circuit in response to the rising or falling edge at the beginning of each period of the second PWM signal, disregarding extremely slight accidental time differences.
In each of the first and second power switching circuits, an overshooting or undershooting variation caused by ringing or the like appears on the rising or falling edge of the first or second PWM power signal whenever the switching operation is performed in response to the first or second PWM signal. The overshooting or undershooting variation thus appearing on the rising or falling edge of the first or second PWM power signal results in undesirable noise.
Under such circumstances, when the switching operation of the first power switching circuit in response to the rising or falling edge at the beginning of each period of the first PWM signal is performed at substantially the same time as the switching operation of the second power switching circuit in response to the rising or falling edge at the beginning of each period of the second PWM signal, disregarding extremely slight accidental time differences as described above, one of the noise resulting from the overshooting or undershooting variation appearing on the rising or falling edge of the first PWM power signal in the first power switching circuit and the noise resulting from the overshooting or undershooting variation appearing on the rising or falling edge of the second PWM power signal in the second power switching circuit is substantially superimposed upon the other of them so as to generate relatively large noise. It is feared that the relatively large noise thus generated in the first and second power switching circuits exerts a bad influence upon a load portion including the first and second power LPFs and the speaker portion, is transmitted to the outside as unwanted radiation so as to disturb communications on the outside and further exerts a bad influence upon electronic apparatus used on the outside.
In particular, in the case of a multi-channel acoustic apparatus in which a plurality of couples of power switching circuits each corresponding to the first and second power switching circuits are provided for multi-channel signals, respectively, it is feared that interference, bad influence and so on to the outside which results from the noise generated by the switching operation in each couple of the power switching circuits may grow worse.